utils/loading_cache.hh: use intrusive list to store the lru entry
utils/loading_cache.hh: use intrusive list to store the lru entry

Fix the shrink() O(n log n) complexity issue by constantly pushing the corresponding intrusive
list entry to the head of the list every time the values are read.

This will keep the list ordered by the last read time from the most recently read
to the least recently read entry.

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@scylladb.com>

scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla